Membrane potentials & excitable cells

Plasma membranes

Separate the intracellular fluid from the extracellular fluid in human cells

Lipids, proteins, cholesterol

Main structural components of PMs (3):

Phospholipids

The majority of the lipids in the plasma membrane are this CLASS:

Phosphatidylcholine

The most abundant form of phospholipid present in the PM:

Amphipathic

Molecules that are part polar and part non-polar

Semi-permeable

Phospholipid membranes are this because they exhibit different permeability to different substances

O2, CO2, N2, benzene

Examples of small hydrophobic molecules that experience HIGH permeability through the lipid bilayer (4):

H2O, glycerol, ethanol

Examples of small, uncharged polar molecules that experience LOW permeability through the lipid bilayer: (3)

Amino acids, glucose, nucleotides

Examples of large, uncharged polar molecules that experience VERY LOW permeability through the lipid bilayer (3):

Ions

A class of molecules that experience the lowest permeability through the lipid bilayer without assistance from protein transporters

Entropy

Randomness

second law of thermodynamics

States that the amount of entropy within a closed system can only increase or remain unchanged if maxed out; introducing order would require additional energy into the system

Diffusion

The natural tendency for particles to move from areas of high concentration to low concentration until they are evenly spread out

Gradients

Concentration or charge differences that drive the movement of molecules from one area to another, often with the driving force proportional to the difference

No net flow

What occurs when a gradient has been resolved

Flow in = flow out

What does it mean to have no net flow or flux?

Electrically excitable

Although all living cells have electrical properties, muscle and nerve cells are this:

Membrane potential

Difference in electrical charge between the inside and outside of the plasma membrane across all cell types in the body

Electroneutral

For every positive charge, there is a negative charge; the bulk of the ECF and ICF is this, even though there is a membrane potential

Tight ring of positive charge outside, negative charge inside

How does a membrane potential form if the majority of the ICF and ECF are electroneutral?

Electrical resistance

The phospholipid bilayer has this property, which make it difficult for ions to pass through the hydrophobic interior

Capacitator

Stores charge; property of the phospholipid bilayer

Current

The movement of ions/charge across the lipid bilayer; THE RATE OF ION FLUX

Open ion channels

How does current conduct across the lipid bilayer?

membrane potential = current x resistance

Ohm’s law

Conductance

Describes the amount of ion that can flux across a membrane during a given amount of time at a particular action potential

A measure of ion channel openness; inverse of resistance

Resistance

Ion channel closed-ness; inverse of conductance

-70 mV

What is the typical resting membrane potential of a neuron?

Negative

What is the charge on the inside of a neuron relative to the outside?

Current flow

Membrane potentials change only when this happens in the form of ions moving across open ion channels

Membrane permeability, ion concentration difference

The magnitude of the membrane potential for a cell depends on (2):

Ion transporters

Membrane proteins that use either ATP or another chemical gradient to create concentration gradients across cell membranes

Na+/K+ ATPase

An ion transporter that uses ATP hydrolysis to pump 3 Na+ OUT of the cell in exchange for 2K+ INTO the cell; important for maintaining resting membrane potential

3Na+

What does the Na+/K+ ATPase pump out of the cell?

2K+

What does the Na+/K+ ATPase pump into the cell?

10 mV

How much of the resting membrane potential can be attributed to the actions of the Na+/K+ ATPase?

Electrochemical gradient

the combined force of a chemical concentration difference and an electrical charge difference across a cell membrane

Concentration, electrical

Two forces that determine the direction of ion flux and establishes the electrochemical gradient for each ion

Na+, K+, Cl-, Ca2+

Major ions involved in membrane potential changes (4):

Ion transporters

Difference in ion concentrations between the ECF and ICF are set up by:

Ion selective pore

Membrane channels that allow for the passage of a very specific ion (e.g. sodium, potassium, etc.) passively through the membrane

No net flux

A phenomenon that occurs wherein a concentration gradient in one direction and electrical gradient in the other cancel each other out; does not mean that there is no flow of an ion at all, just that the flow inwards equals the flow outwards

No change

What happens to the membrane potential when it is in a state of no net flux?

Equilibrium potential

The electrical charge that exists across a membrane when an ion achieves no net flux

Nernst equation

The equation that is used to calculate the equilibrium potential for any ion

Valence

Charge of an ion

Towards equilibrium potential

Which direction does net flux always occur? To bring the membrane potential…

Conducting pore

Feature of ion channels that allows them to select for a particular ion or class of ions to flux across the membrane

Gate

A part of an ion channel that can open and close the conducting pore

Sensors

Part of ion channel that controls its openning or closing, often in response to stimuli like membrane potential changes, ligands, etc.

Mechanical-gated channels

Ion channels that open/close in respond to the stretch of the plasma membrane

open, closed, inactive

What states can ion channels be in? (3)

-80 to -90 mV

What is the resting membrane potential in cardiac and skeletal muscle cells?

Na+, K+

What are the two main ions involved in determining a cell’s resting membrane potential?

Leak channels

non-gated, constantly open protein pores in cell membranes that allow specific ions to drift down their electrochemical gradients

K+

Do neurons and muscle cells have more K+ or Na+ leak channels?

K+

Which equilibrium potential is the resting membrane potential of muscle and neurons closer to? K+ or Na+?

-90 mV

What is the equilibrium potential of K+?

+60 mV

What is the equilibrium potential for Na+?

balanced

In the steady state resting cell, the leak of K+ IN and Na+ out are:

Depolarizing

Cell’s membrane potential becomes more positive from its resting membrane potential

Repolarizing

Return of a cell’s membrane potential to the resting membrane potential, usually from a depolarized state

Hyperpolarizing

Polarization of a membrane’s membrane potential more negative than its resting membrane potential

Action potential

Unitary electrical signals in excitable cells involving brief, explosive, large changes in membrane potentials that can occur at high frequencies and travel long distances along axons or other excitable cell membranes

Shape, magnitude, duration

Action potentials can differ in their: (3)

All or nothing

A feature of action potentials in which so long as a certain threshold is achieved, the neuron or muscle cell will fire

Refractory periods

Time immediately following action potential firing during which cells are unresponsive to further stimulation that tries to elicit a second action potential

Threshold

The voltage at which an action potential fires 50% of the time

Voltage gated Na+ channels

Which ion channels usually determine the threshold for action potential in neurons and muscle cells?

Permeability

Proportion that describes the ease with which a substance fluxes across a membrane

Number of open channels

For ions, permeability is proportional to:

0

What is the current when there is no net flux across a membrane?

Driving force

The difference between membrane potential and the equilibrium potential

Selective permeability, gating

Action potentials require these two key properties of ion channels:

voltage-gated Ca+ channels

In heart muscles, these channels are also involved in action potentials on TOP of the Na+ and K+ channels

Rapidly

Voltage gated Na+ open ________ and depolarize the membrane

Slowly

Voltage gated K+ channels open ___________ and are responsible for repolarizing the membrane

Arginine, lysine

These positively charged amino acids are VOLTAGE SENSORS that detect changes in membrane potential and change conformations of voltage gated ion channels

open

Conductance increases and resistance decreases when ion channels are ________

Na+

What ion is the driving force behind the DEPOLARIZATION phase of the action potential? I.e. which Eion is the membrane potential going towards?

K+

What ion is the driving force behind the POLARIZATION phase of the action potential? I.e. which Eion is the membrane potential going towards?

influx

At resting membrane potential, neurons experience Na+ influx/efflux?

efflux

At resting membrane potential, neurons experience K+ influx/efflux?

current of K+, Na+

these are equal at the resting membrane potential due to high permeability of K+; allowing the resting potential to stay consistent around -70 mV

Na+

Which ion has the greater driving force at resting membrane potential - Na+ or K+?

Chord conductance equation

Calculates the membrane potential when there are membranes that are permeable to more than one ion

30 mV

What voltage does the peak of action potential usually reach?

voltage gated Ca2+ channels

These channels are also involved in action potentials in cardiac muscle in addition to the voltage gated K+ and Na+ channels

Slowly

Voltage gated Ca2+ channels open and inactivate _________ in cardiac muscle cells

Initiating, prolonging

Voltage gated Ca2+ channels are responsible for _____ and _____ action potentials

Depolarize

Voltage gated Ca2+ channels do this to the voltage of cardiac cells during action potentials (what kind of polarization?)

Opening of ion channels

What is the rate limiting step for action potentials?

Absolute refractory period

Period during which the cell absolutely cannot undergo another action potential regardless of the strength of the stimulus; all Na+ channels inactivated

Relative refractory period

A period during which an action potential can be induced only if there is a stronger than normal stimulus applied to the cell; some Na+ channels transitioned from inactive to closed, so threshold is elevated as fewer available to participate in AP

0 mv

The amplitude of the action potential produced by a neuron in its RELATIVE refractory period is reduced to about: